Subject: Re: 3.3.0, 3.4-rc1 reproducible tun Oops

From: Simon Kirby <[email-masked]>
Date: Fri, 18 May 2012 18:07:43 -0700

On Wed, Apr 18, 2012 at 03:32:27PM +0400, Stanislav Kinsbursky wrote:

> 17.04.2012 22:35, Simon Kirby ??????????:
> >On Tue, Apr 17, 2012 at 04:18:53PM +0400, Stanislav Kinsbursky wrote:
> >>
> >>Hi, Simon.
> >>Could you please try to apply the patch below on top of your the
> >>tree (with 1ab5ecb90cb6a3df1476e052f76a6e8f6511cb3d applied) and
> >>check does it fix the problem:
> >>
> >>diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> >>index bb8c72c..1fc4622 100644
> >>--- a/drivers/net/tun.c
> >>+++ b/drivers/net/tun.c
> >>@@ -1540,13 +1540,10 @@ static int tun_chr_close(struct inode
> >>*inode, struct file *file)
> >>  			if (dev->reg_state == NETREG_REGISTERED)
> >>  				unregister_netdevice(dev);
> >>  			rtnl_unlock();
> >>-		}
> >>+		} else
> >>+			sock_put(tun->socket.sk);
> >>  	}
> >>
> >>-	tun = tfile->tun;
> >>-	if (tun)
> >>-		sock_put(tun->socket.sk);
> >>-
> >>  	put_net(tfile->net);
> >>  	kfree(tfile);
> >
> >(Whitespace-damaged patch, applied manually)
> >
> >Yes, I no longer see crashes with this applied. I haven't tried with
> >kmemleak or similar, but it seems to work.
> >
> >Thanks,
> >
> 
> This bug looks like double free, but I can't understand how does this can happen...
> Simon, would be really great, if you'll describe in details some
> simple way, how to reproduce the bug.

Oh, sorry, I did not see this until now. I just noticed it was still
floating in my tree with no upstream changes yet, then found your email.
I still have not seen any issues since applying your patch.

I was definitely seeing the issue on 3.4-rc3. I can try and see if it
still occurs with your patch removed, if that would help.

Do you have a box on which you can set up an SSH tunnel? In my case, I
can reproduce it easily with three boxes. From home, I run ssh to my work
box to establish the layer 2 tunnel. This goes through a ProxyCommand to
jump through an entry box, but I don't think that should matter. I use a
cheap tunnel start script similar to this:

work_net=10.0.0.0/8
work_tun_ip=10.x.x.x
home_tun_ip=10.x.x.x
echo 1 > /proc/sys/net/ipv4/conf/eth0/proxy_arp
ssh -w any:any <work box> "ifconfig tun0 $work_tun_ip pointopoint
$home_tun_ip; echo 'ifconfig tun0 $home_tun_ip pointopoint $work_tun_ip
&& ip route add $work_net via $work_tun_ip'; sleep 1d" | sh -v

...there's probably a better way, but it works. To reproduce, I log in
to a third box over this tunnel, and start a "vmstat 1", so that packets
keep coming back to the tunnel host. ^C on the SSH session will then
produce an Oops within a second.

With CONFIG_SLUB_DEBUG=y and booting with slub_debug=FZPU, I got the
Redzone overwritten notice. Without it, the box usually Oopses and
hangs immediately. Sometimes, I might have to reconnect the tunnel and
^C it once more. If I don't have that vmstat session open, it usually
doesn't crash.

Does this work for you?

Simon-